The invention is related to the field of wireless communications, and in particular to an iterative algorithm to calculate precoders in a multi-user downlink multiple-input and multiple-output (MIMO) system.
The precoding schemes in long term evolution (LTE) can be codebook-based or non-codebook based. For codebook-based precoding, the UE (mobile receiver unit) estimates the channel using cell-specific reference signals (CRS) and feedbacks the precoder matrix index (PMI) and Rank Index (RI) to the base station (BS). The BS can choose to apply such precoder. For non-codebook-based precoding, the BS needs more accurate channel state information (CSI). (e.g., in time division duplex (TDD), CSI can be obtained using uplink reference signals.) The transmission modes 7, 8, and 9 can support non-codebook-based precoding with single-layer, two layers, and eight layers transmission, respectively
The detailed procedures of non-codebook-based precoding are described as follows. Considering the time division duplex (TDD) mode, to design the non-codebook-based precoder, the BS uses the uplink pilots called sounding reference signals (SRS) to estimate the uplink CSI. Due to the channel reciprocity in TDD, the BS can obtain the downlink CSI using uplink CSI, and design the corresponding precoder. The UEs do not need to be informed about this precoder since the UEs can estimate the equivalent channel (the composite effect of the precoder and the channel) with the presence of demodulation reference signals (DM-RS, or dedicated reference signals, DRS), which is inserted before precoding. Therefore, the receive beamforming vector (for single data stream) or matrix (for multiple data streams) can be accordingly designed by the UEs.
Other non-codebook-based precoding designs use CRS or DRS. The DRS design has the UE (mobile receiver) use the DRS to estimate the equivalent channel as described above, and designs the receive beamformer. The CRS design has the UE use the CRS to estimate the CSI, which does not contain the effect of the precoder. In this case, the receiver can only design the receive beamfomer according to the channel. Intuitively, the CRS based scheme provides less information to the UE than the DRS based scheme. Thus it is expected that the DRS based scheme should perform better than the CRS based scheme. Thus, non-codebook-based precoding can achieve better performance, but requires extra signaling and computational overhead.